AND8135/D Efficient High Power Flash Light Prepared by: Michael Bairanzade ON Semiconductor http://onsemi.com APPLICATION NOTE INTRODUCTION Depending upon the type of flash involved, the amount of energy stored into capacitor C1 can be a low 10 Joule (small camera) to thousand of Joule for professional applications. Although the xenon lamps dominate the standard camera market, they are not applicable in the portable phone. As a matter of fact, the large reservoir capacitor and high voltage associated with the xenon flash make such a concept not suitable for the cellular phone equipments. The semiconductor based White LED devices provide the right choice when limited flash light becomes necessary to illuminate a photographic scene. This paper depicts the basics of the xenon concept and details a typical White LED flash application. +260 V XENON TUBE 3 TRIGGER C2 100 nF/ 300 V A low pressure of a rare gas mixture fills a glass envelope with two ends electrodes on both sides. In steady state, the voltage across the electrodes is set to a value well below the trigger voltage as depicted Figure 1. At this point, no current flows and the system is stable until a trigger voltage is applied to the third electrode. This high voltage pulse, in the 1 kV range, comes from a transformer built with a small magnetic tore triggered by an abrupt discharge of the capacitor C2 (see Figure 2). T1 + C2 4.7 F/ 300 V 2 GND Figure 2. Basic Xenon Flash The net advantage of such a concept is the very short pulse, making easy snap shot photos to capture moving stuffs. The drawbacks are the large physical size of the reservoir capacitor and the recycle time needed to recharge the capacitor between two shots (in the 5 sec range for consumer applications). Clearly, these drawbacks make the xenon based flash not suitable for hand held cellular phone, with limited size and energy supply. I WHITE LED FLASH V To overcome the physical size limitation, the flash concept is to make profit of the high efficiency, in term of light, coming from the modern white LED. With a 4 V forward drop voltage, such diodes do not need extra high voltage trigger pulse, they are extremely fast to turn ON/OFF and all the associated electronic circuit can be housed inside a standard portable phone. Since the white LED have electrical characteristics similar to the standard LED (see Figure 3), one must provide a constant forward current to control the device. VCC Vtrig Figure 1. Xenon Flash Breakdown Voltage The gas is ignited and the plasma generates a bright flash, the typical duration being 2 ms for consumer applications. October, 2003 − Rev. 0 X1 S1 XENON LAMP CONCEPT Semiconductor Components Industries, LLC, 2003 1 R1 470 k 1 Publication Order Number: AND8135/D AND8135/D The circuit, built around the NCP5007, is designed to support both the low beam current and the high flash pulse as requested during the capture of a photo. The DC/DC boost converter, associated to the sense resistor R1, provides a constant current to the load to properly bias the white LEDs. With an internal 200 mV voltage reference (Vref), the chip minimizes the drops along the battery supply path. IF 100 mA 10 mA −5 V VF VRR 3.5 V −1 A 4.0 V Low Power Beam Operating Mode Generally speaking, this mode of operation is used to pre−light the scene to be capture in order to minimize the red−eye effect. The NMOS transistor Q1 is biased OFF and R1 provides the feedback voltage to regulate the load current. The value of R1 is derived from the Ohm’s law: IR Figure 3. Typical White LED Characteristics Consequently, a standard voltage source cannot be used straightforward and an extra ballast is necessary to set up the current. On the other hand, the flash must be capable to operate over the typical battery voltage spread (2.8 V to 5.2 V) and a more suitable structure than a simple linear voltage regulator is mandatory. To achieve such constraints, ON Semiconductor has developed a full family of white LED drivers, among which the NCP5007 can fulfill the flash application demands. V R1 ref Iout With a typical 4 mA operating bias of the LED during the illumination of the scene, the sense resistor is 51 Ω. The current can be dynamically modulated, if necessary, by using the EN signal pin 3 as a digital control: such a mode of operation is depicted in the NCP5007 data sheet. The same pin can be used to control the DC/DC by a bit from the external CPU. Of course, a more powerful light can be provided by setting the sense resistor accordingly. From a practical stand point, capacitor C2 is mandatory to avoid large spikes during the energy transfers from the inductor L1 and the white LEDs. Moreover, such a capacitor smoothes the current flowing into the LEDs, yielding a better light efficiency. TYPICAL FLASH APPLICATION Since the battery voltage ranges from a low 2.8 V to a high 5.2 V, the simplest and economic way to handle this span is to arrange the white LED in series as depicted Figure 4. Such a layout avoid the leakage current during the stand by mode operation (most of the time, the flash is not activated!). VBat U1 3 ENABLE GND 2 1 EN VBat C1 4.7 F 5 L1 22 H Vout FB GND D1 GND 4 D4 D3 D2 R10 3.3 GND Q1 FLASH C2 10 F/ 16 V MBR0530 NCP5007 LWT67C LWT67C LWT67C R1 51 GND Figure 4. Typical Portable White LED Flash Circuit http://onsemi.com 2 AND8135/D High Power Flash Mode an external resistor in series as depicted in the demo board schematic diagram. Table 1 gives a selection of the preferred product to handle such a function. Once the system is ready to take the photo, the flash is activated by forcing the high current bias into the white LEDs. Unlike the silver film, the electronic sensor of a digital camera cannot capture the scene is a couple of millisecond, but a much longer delay is necessary to save all the pixels. Typically, such a delay ranges from 100 ms to 200 ms, depending upon the type of camera and lens. Consequently, using a reservoir capacitor to supply the large current during 200 ms is not really feasible. In fact, to sustain 100 mA during 200 ms, with three LEDs in series, assuming the voltage cannot drop more than 0.5 V during the pulse, one should have a 40000 µF/16 V electrolytic capacitor, a value not compatible with a portable equipment. On top of that, to re−charge this capacitor in a reasonable time, typically one second, the DC/DC converter should provide around 500 mA when loading the capacitor from zero. Therefore, instead of designing a chip to re−charge a capacitor, it is far better to use the converter to supply immediately the current called by the application. To activate the flash, one shall turn ON the NMOS Q1, providing a lower sense resistor in the feedback loop. The NMOS is selected with an internal Rdson suitable for the expected current. Since the Rdson of the NMOS varies largely with the temperature and the spread from one lot to another one is relatively large, one can get a more predictable circuit by using a larger NMOS associated with Table 1. Preferred NMOS Products Part Icmax Rdson Package BVSS MMBF0201NLT1 6A 35 mΩ SO−8 30 V MMBF0201NLT1 0.2 A 1Ω SOT−23 20 V MMBF2201NT1 0.2 A 1Ω SC−70 20 V NTA4001NT1 0.24 A 1.5 Ω SC−75 20 V MMFT960T1 0.3 A 1.7 Ω SOT−223 60 V One can use a NPN bipolar device to fulfill this function, but the saturation voltages (Vcesat) of such devices is close to the 200 mV define by the internal reference, and cannot be easily implemented in this type of circuit. DEMO BOARD SCHEMATIC DIAGRAM The demo board, depicted in Figure 10, supports the low beam and the high power flash, together with a digital PWM circuit to dim the LED. On top of that, a built−in clock provides the capability to generate multiple flash for evaluation purpose. The mode of operation is selected by means of switches S1 and S2, associated to potentiometers P1 to P4 as depicted Table 2. The third switch S3 is a push button to manually trig the flash. Table 2. Switches Configurations and Potentiometers Functions S1 Select the NCP5007 mode of operation: GND = EN pin 3 forced to High, DC operation VCC = EN pin 3 pulsed If S1 = VCC, then dim the light out of the LED : P1 = Adjust the pulse width applied to EN pin 3 P2 = Adjust the PWM frequency S2 Select the Power Flash mode of operation: GND = Single shot triggered by S3 VCC = Repetitive mode If S2 = VCC, then P3 = Adjust the power flash repetitive frequency P4 = Adjust the power flash duration S3 Manual switch to trig the power flash The system can be re−arranged to either dim the light when running the low beam current, or to generate a pulsed flash at a low pace. The dimming function is activated when switch S1 is High. In this mode, the clock built with gates U3, associated with the one shot circuit U1, controls the EN pin 3, thus a PWM modulation of the DC load current. The pulsed flash is activated when switch S2 is High. In this mode, the clock built with gates U3 and U4 , associated with the second side of the one shot U1, provide a low rate to trig the flash. The pulse width can be manually adjusted with potentiometer P4. The waveforms captured from the demo board (see Figure 5 to Figure 9), illustrate the currents and voltages across the major points. The extra functions make possible the operation of the demo board on a stand alone basis, without any need from external control. However, in order to provide higher flexibility, provisions are made to connect the demo board to a MPU: the three pins connector shall be connected to the appropriate port to control the LED dimming and flash functions. In the stand alone operation, switches S1 and S3 are forced to Low and the chip runs continuously. The low beam current is set up by the sense resistor R1: the demo board comes with a 51 Ω resistor, yielding 4 mA of DC current through the white LED. At this point, one can trig the flash by pushing switch S3. The flash is pre−set to provide 60 mA during the 200 ms time adjusted by potentiometer P4. http://onsemi.com 3 AND8135/D Figure 5. Low Beam Inductor Current Figure 6. High Power Flash Inductor Current http://onsemi.com 4 AND8135/D Figure 7. Input Supply Current Figure 8. Low Beam & High Beam Output Current http://onsemi.com 5 AND8135/D Figure 9. Combined Low Beam & Power Flash Output Current & Output Power http://onsemi.com 6 AND8135/D C6 4.7 F/16 V C2 4.7 F/6 V GND 2 1 CLR Q 5 12 Q 9 TRA 10 11 TRB 6 7 CTC RCCOM Adjust Flash Pulse Width U1B M54HC123 SNJ54HC132J (14) 10 U3C 1N4148 1N4148 NL27WZ14 4 U4B 3 13 EXTL_EN 6 U4A 11 R8 100k GND R14 1.5k GND GND VCC REPEAT TRIG GND Figure 10. Demo Board Schematic Diagram http://onsemi.com GND J2 EXTERNAL 3 2 1 S3 GND 7 1 R6 10k P3 100kA U3D Adjust Flash Duty Cycle 10 F/10 V NL27WZ14 C4 EXTL_FLASH 1 nF SNJ54HC132 C7 R3 10k D9 PWM VCC D8 MBR0530 GND GND S1 NORMAL/PWM R13 1.5k VCC D2 D7 NORMAL: Constant DC Current PWM: dim LED intensity 4.7 F/16 V GND VCC GND SNJ54HC132 5 GND SNJ54HC132 6 U3B 4 3 U3A 1 R7 100k 2 Adjust PWM R2 10k 100 nF C9 J1 Vbat D6 VCC P1 500kA C10 9 R4 10k P2 100kA C8 1F/10V GND 8 VFB GND 100 nF C11 6 R9 51R 100 nF VCC R5 10k 12 2 NL27WZ32 2 4 TP1 C3 R1 10k P4 500kA GND GND Q CLR U1A M54HC123 1 C1 VCC Q1 MMBF0201NLT1 VCC 100 nF 5 3 GND 4 Vout 1 FB R10 3.3R 1 TRA 2 TRB 3 14 15 CTC RCCOM Q 13 U5A 7 3 ENVbat U2 NCP5007 5 D3 LWT67C U5B NL27WZ32 D4 LWT67C VCC GND GND GND D5 LWT67C MBR0530 D1 L1 22 H R11 10k R12 10k C5 4.7 F/16 V S2 SINGLE/ REPEAT AND8135/D Figure 11. PCB Layout http://onsemi.com 8 AND8135/D Table 3. High Beam Demo Board Part List Used Part Designator Footprint Description 2 1.5 kΩ R13, R14 0805 Resistor 1 3.3 Ω R10 0805 Resistor 1 51 Ω R9 0805 Resistor 8 10 kΩ R1, R2, R3, R4, R5, R6, R11, R12 0805 Resistor 2 100 kΩ R7, R8 0805 Resistor 2 1N4148 D6, D7 DIODE0.4 Diode 2 100 kΩ P2, P3 VR4 Potentiometer, Linear 1 500 kΩ P1 VR4 Potentiometer, Linear 1 500 kΩ P4 VR4 Potentiometer, Linear 4 100 nF C1, C3, C9, C10 0805 Ceramic Capacitor, MURATA 1 10 µF/10 V C4 1210 Ceramic Capacitor, MURATA 1 1 µF/10 V C8 0805 Ceramic Capacitor, MURATA 1 1 nF C7 0805 Ceramic Capacitor, MURATA 4 4.7 µF/6 V C2, C5, C6, C11 1210 Ceramic Capacitor, MURATA 2 LED D8, D9 LED_2 LED 3 LED D3, D4, D5 LED_2 LED: OSRAM LWT67SQ2−4 1 22 µH L1 1210 Inductor: CoilCraft 1008 1 M54HC123 U1 SO−16 Dual Retriggerable one shot 2 MBR0530 D1, D2 1210 Schottky Diode 1 SNJ54HC132 U3 SO−14 Quadruple Positive−NAND Gate with Schmitt−Trigger Input 1 MMBF0201NLT1 Q1 SOT−23 MOSFET 1 NCP5007 U2 TSSOP5 White LED driver 1 NL27WZ14 U4 SOT_23B Dual schmitt trigger inverter 1 NL27WZ32 U5 US8 Dual OR gate 1 EXTERNAL J2 SIP3 Connector 1 GND Z1 GND_TEST Connector 1 NORMAL/PWM S1 SIP3 Manual Switch 1 SINGLE/REPEAT S2 SIP3 Manual Switch 1 TRIG S3 PUSH_BUT_B Push Button 1 VFB TP1 TEST_POINT Connector 1 Vbat J1 PLUG_4MM_DUAL Connector http://onsemi.com 9 AND8135/D ON Semiconductor and are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). 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